Spark plug having heat- and corrosion-resistant surface

Abstract

A metal body has a heat- and corrosion-resistant surface which is composed of a layer of nickel plating applied to a surface of a metal base and an electrolytic chromate film formed over the nickel plating. Preferably, the chromate film has a thickness of 3 μm or less. The chromate film may have a color imparted upon its formation by an electrolytic chromate treatment. A metal sheel of a spark plug may be mentioned as one example of the metal body.

What is claimed is: 
     
       1. In a spark plug including a metal shell and a threaded portion adapted to threadedly secure the spark plug to an engine head of an internal combustion engine, the metal shell and the threaded portion each having an outer surface, the improvement comprising a heat-resistant and corrosion-resistant surface on the outer surface of the metal shell, said heat-resistant and corrosion-resistant surface comprising a layer of nickel plating on said outside surface of the metal shell and an electrolytic chromate film formed over said layer of nickel plating. 
     
     
       2. The spark plug as claimed in claim 1, wherein said metal shell includes a base made of steel defining a portion of said outer surface of the metal shell. 
     
     
       3. The spark plug as claimed in claim 1, wherein the layer of nickel plating has a thickness in the range of from 3 μm to 8 μm and the chromate film has a thickness not greater than 3 μm. 
     
     
       4. The spark plug as claimed in claim 3, wherein said heat-resistant and corrosion-resistant surface is also on the outer surface of the threaded portion. 
     
     
       5. The spark plug as claimed in claim 1, wherein the chromate film has a color imparted upon its formation by an electrolytic chromate treatment. 
     
     
       6. The spark plug as claimed in claim 1, wherein the chromate film has a thickness of 0.1 t 2 μm. 
     
     
       7. The spark plug as claimed in claim 1, wherein the layer of nickel plating has a thickness in the range of 3 μm to 8 μm.